1. Field of the Invention
This invention relates to a method and system for automated detection of clustered microcalcifications from digital images without reduction of radiologist sensitivity.
2. Discussion of Background
Mammography, along with physical examination, is the current procedure of choice for breast cancer screening. Screening mammography has been responsible for an estimated 30 to 35 percent reduction in breast cancer mortality rates. However, in 1996 approximately 185,700 new breast cancer cases were diagnosed and 44,300 women died from this disease. Women have about a 1 in 8 chance of being diagnosed with breast cancer, and 1 in 30 will die of this disease in her lifetime.
Although mammography is a well-studied and standardized methodology, for 10 to 30 percent of women diagnosed with breast cancer, their mammograms were interpreted as negative. Additionally, only 10 to 20 percent of patients referred for biopsy based on mammographic findings prove to have cancer. Further, estimates indicate the malignancies missed by radiologists are evident in two-thirds of the mammograms retrospectively. Missed detections may be attributed to several factors including: poor image quality, improper patient positioning, inaccurate interpretation, fibroglandular tissue obscuration, subtle nature of radiographic findings, eye fatigue, or oversight.
To increase sensitivity, a double reading has been suggested. However, the growing increase in the number of screening mammograms makes this option unlikely. Alternatively, a computer-aided diagnosis (CAD or CADx) system may act as a "second reader" to assist the radiologist in detecting and diagnosing lesions. Several investigators have attempted to analyze mammographic abnormalities with digital computers. However, the known studies are believed to have achieved rates of true-positive detections versus false-positive detections that are undesirably low.
Microcalcifications represent an ideal target for automated detection because subtle microcalcifications are often the first and sometimes the only radiographic findings in early, curable breast cancers, yet individual microcalcifications in a suspicious cluster have a fairly limited range of radiographic appearances. Between 30 and 50 percent of breast carcinomas detected radiographically demonstrate microcalcifications on mammograms, and between 60 and 80 percent of breast carcinomas reveal microcalcifications upon microscopic examination. Any increase in the detection rate of microcalcifications by mammography will lead to further improvements in its efficacy in the detection of early breast cancer.
Although the promise of CAD systems is to increase the ability of physicians to diagnose cancer, the problem is that all CAD systems fail to detect some regions of interest that could be found by a human interpreter. However, human interpreters also miss regions of interest that are subsequently shown to be indicators of cancers. Missing a region that is associated with a cancer is termed a false negative error while associating a normal region with a cancer is termed a false positive error.
It is not yet clear how CAD system outputs are to be incorporated by practicing radiologists into their mammographic analyses. No existing CAD system can claim to find all of the suspicious regions detected by an average radiologist, and they tend to have unacceptably high false positive error rates. However, CAD systems are capable of finding some suspicious regions that may be missed by radiologists.
Current CAD systems operate with high false positive readings. These false positive detections reduce the usefulness of the device since the human operator must scrutinize the mammograms at each location indicated by the CAD device. Therefore, methods are required to reduce the instances of false positive detections.
Detection of clusters of microcalcifications is important since they are exhibited in 30-50% of breast cancer cases. Individual microcalcifications are typically seen as locally bright spots, ranging in size from 0.1 to 0.3 millimeters. When three to five microcalcifications are observed in a one square centimeter region of a mammogram, many radiologists recommend a biopsy of the region, due to the known association of such clusters with cancer. When associated with cancer, there is no regular geometric distribution of the individual microcalcifications in a cluster. Unfortunately, current CAD systems are observed to indicate clusters of microcalcifications along veins, milk ducts and other elongated structures. Frequently, this is due to calcium or other deposits within the structures which look radiographically similar to microcalcifications. When multiple such deposits are detected within a small portion of the breast, a CAD system may falsely indicate a suspicious region. Human observers discount such detections by noting that the microcalcifications are within a vein or duct.